Mathematica usually works by taking whatever input you give, and then processing it. Sometimes, however, you may want to have a program you write explicitly request more input. You can do this using Input and InputString.
|Input||read an expression as input|
|InputString||read a string as input|
|Input["prompt"]||issue a prompt, then read an expression|
|InputString["prompt"]||issue a prompt, then read a string|
Exactly how Input and InputString work depends on the computer system and Mathematica interface you are using. With a text-based interface, they typically just wait for standard input, terminated with a newline. With a notebook interface, however, they typically get the front end to put up a "dialog box", in which the user can enter input.